1. Field of the Invention
The present invention relates generally to inkjet printhead design and operation, and in particular to DOD (Drop-On-Demand) inkjet printheads.
2. Description of Related Art
Various types of DOD inkjet printhead nozzles are known in the art. See, for example, U.S. Pat. No. 5,808,639 to Silverbrook.
One type of inkjet, commonly referred to as xe2x80x9cbubblejetxe2x80x9d, features a printhead having a plurality of nozzles. In a typical bubblejet printhead, each nozzle is connected to a separate ink chamber in the printhead, and each chamber is provided with a heater element and filled with ink. To eject a drop of ink from the nozzle, the heater element in the chamber is energized until ink in the chamber near the heater element vaporizes to form a bubble, which increases pressure on the ink in the chamber and expels ink from the nozzle. See for example U.S. Pat. No. 5,841,452 to Silverbrook. Disadvantages of this approach include, for example, relatively large energy requirements and cumulative damage to the printhead caused by mechanical shock when the bubble cools and collapses. In addition, specialized inks must be used that are capable of enduring this thermal cycling without losing inking properties, forming undesirable residues within the chamber, and so forth.
In another type of inkjet mechanism, each nozzle in a printhead has one or more piezoelectric transducers arranged in a chamber or passageway that transports ink to the nozzle. When a piezoelectric transducer corresponding to a nozzle is activated with an electrical pulse of appropriate voltage and duration, it generates pressure pulse in the ink that expels a drop of ink from nozzle. Thus, the printhead can be operated by selectively activating piezoelectric transducers corresponding to various nozzles in the printhead. See for example U.S. Pat. No. 4,992,808 to Bartky, et al. However, it can be difficult and expensive to manufacture such a printhead with a high nozzle density due to the small size of the piezoelectric transducers.
In yet another type of inkjet mechanism, ink is supplied to nozzles in a printhead at a constant pressure. Viscosity and surface tension of the ink allow the ink to fill each nozzle, but are sufficient to prevent the ink from exiting the nozzle. Each nozzle is equipped with an electrically activated heater. Ink droplets are released from the printhead by selectively activating nozzle heaters to provide heat sufficient to lower the viscosity and surface tension of the ink and allow the ink to leave the nozzles. See for example U.S. Pat. No. 4,164,745 to Cielo, et al. and U.S. Pat. No. 5,812,159 to Anagnostopoulos, et al. However, to function effectively this technique can require inks that have a large change in viscosity and surface tension per unit of temperature change. In addition, it is less well suited to situations where the print medium is located at a distance from the printhead so that an ink drop must detach from the printhead and travel ballistically, instead of simply extending from the nozzle to contact the print medium (as shown for example in FIGS. 1 and 4 of U.S. Pat. No. 4,164,745 to Cielo, et al.).
Accordingly, a need exists for an inkjet printhead that is versatile, robust, simple and inexpensive to manufacture, with a high nozzle density and a short cycle time.
In accordance with exemplary embodiments of the present invention, an ink chamber in a printhead is provided with a plurality of nozzles and at least one piezoelectric actuator for increasing pressure of ink within the chamber. Each nozzle is equipped with a heater element, and the number of nozzles exceeds the number of piezoelectric actuators. Having multiple nozzles per piezoelectric actuator instead of one nozzle per piezoelectric actuator reduces manufacturing costs and increases simplicity and reliability of the printhead. This is because larger piezoelectric actuators are easier to manufacture, install and operate, and fewer piezoelectric actuators are required in the printhead. In addition, using larger and fewer piezoelectric actuators allows larger ink chambers to be formed within the printhead, which reduces or eliminates ink clogs in the printhead and eases clearing of clogs that do occur.